libre is a generic library for real-time communications with asynchronous I/O support. It is written in portable POSIX source code that conforms to the ANSI C89 and ISO C99 standards. It is robust and fast, with a low memory footprint. It also features RFC compliance and support for IPv4 and IPv6. Protocol implementations include SIP, SDP, RTP/RTCP, BFCP, DNS,
STUN/TURN/ICE, HTTP, and WebSockets.

Discretio is an application (and service) for secure VoIP on smartphones. It allows users to establish calls over the Internet while using state of the art encryption technologies to avoid eavesdropping.

Ekiga (formely known as GnomeMeeting) is a soft phone, video conferencing, and instant messenger application for use over the Internet. It supports HD sound quality and video up to DVD size and quality. It is interoperable with many other standards compliant software, hardware, and service providers as it uses both of the major telephony standards, SIP and H.323.

HOMER is a robust, carrier-grade, scalable SIP capturing system and monitoring application with hEP, IP Proto4 (IPIP) encapsulation, and port mirroring/monitoring support right out of the box, ready to process and store large amounts of signaling with instant searches, end-to-end analysis, and drill-down capabilities for ITSPs, VoIP providers, and trunk suppliers using SIP signaling.

The SIP forwarding daemon (implemented as a
stateless SIP proxy) allows you to seamlessly
forward SIP requests to other SIP servers. Its
main purpose is to enable users to use their own
domain name in SIP URIs without the hassle of
having to run a full-blown SIP server (by
forwarding SIP requests to third-party SIP
servers). Configuration information is stored in
an SQLite database, and low resource consumption
is a main priority for the project.

SIP Inspector is a tool to simulate different SIP messages and scenarios. You can create your own SIP signaling scenarios, customize SIP messages, and monitor incoming and outgoing messages. The tool can play RTP streams from a pcap file.

Billion Softswitch works as a switchboard in SIP/H.323 VoIP networks, proxying both signalling and media streams while performing cross-protocol conversion and even media stream transcoding. It integrates advanced call routing features as well as embedded compact billing system. It supports subscriber registration by SIP Registrar and RAS, call authorization, call data record (CDR) list maintenance, external billing system interfaces, NAT/firewall support, and report generation, and is easy to install and maintain.